The present invention, while of general application, is particularly well suited for disposing of hazardous waste associated with secondary aluminum smelting.
Secondary smelting of aluminum is an environmentally desirable practice as it recycles much of the material that would normally be deposited in the waste stream. The smelting procedure nevertheless produces materials such as cadmium oxide, lead oxide, chromium oxide, nickel oxide and other undesirable residues which have been designated as harmful to the environment.
To collect undesirable residues in the waste gas from aluminum or other metal smelting furnaces, it has been common practice heretofore to direct the furnace emissions to a baghouse dust collector, or in some cases to a scrubber or other filter screening system. As is well known, baghouse dust collectors comprise a series of fibrous filter bags which receive the waste gas stream and remove residues from the stream. The removed residues form a coating on the filter bags which is periodically removed and discarded as hazardous waste as they comprise hazardous materials.
To increase the filtration capability of the bags they often are precoated with fine particles of lime, limestone flour or nepthylene syenite. In addition to reducing the size of the hazardous residues that can pass through the filters, the particles provide a flame or heat barrier and in the case of lime and limestone tend to neutralize any acids in the exhaust.
The precoating materials utilized heretofore, however, have proved deficient in several respects. In the secondary melting of aluminum, for example, aluminum particles in the waste gas stream are oxidizing and burning, and the heat barrier properties of the precoating material have been insufficient to prevent a charring or burning of the filter bags. Baghouse fires are difficult to control and can cause extensive damage. In addition, the prior precoating materials have not been particularly effective in facilitating the removal of some of the more hazardous oxides in the stream. Further difficulties have been encountered in the disposal of the hazardous residues from the bags.